It is often necessary in the design of electronic circuitry to ensure that specific portions of a circuit begin operation in a known state. It is also often true that specific types of circuitry do not operate properly unless the power supply voltage is above a certain critical voltage. A circuit that monitors an analog supply voltage, and determines whether the voltage is sufficient for reliable operation of associated circuitry, is often termed a “brown-out detector.”
Determining whether the supply voltage has reached a sufficient level can be particularly important when battery-powered equipment is involved. In a battery-powered environment, it is also desirable that any monitoring circuitry employed utilize as little power as possible. In addition, a monitoring circuit should function reliably at a predetermined threshold voltage regardless of operating temperature.
It is also a desirable feature that the monitoring circuit respond predictably to process variations. In many applications, it is actually desirable for monitoring circuit operation to vary with certain process parameters, particularly since process variations can actually impact an electronic circuit's performance specifications, particularly insofar as proper operating supply voltage is concerned.
A power-on reset circuit that is designed to sense and respond to power supply voltage level is described in U.S. Pat. No. 6,239,630. The patent describes a power-on reset circuit that uses all-CMOS circuitry to initiate a reset signal when the circuit's supply voltage is low, then terminates the reset signal when the supply voltage exceeds a reference voltage by at least the greater of PFET and NFET threshold voltages. The heart of the circuit is a diode-connected bipolar transistor that establishes the reference voltage. The disadvantages of this approach are that the threshold voltage level is both process and temperature dependent, and resistors are required that must be large when low power operation is desired.
A precision power-on reset circuit is described in U.S. Pat. No. 5,959,477. This reference is directed toward a circuit that is relatively insensitive to temperature and process variations. Although a bipolar transistor base-emitter junction is utilized, and it is known that VBE has a negative temperature coefficient, resistance ratios and device shape factors are varied to compensate for temperature variation to result in a circuit with minimal sensitivity to both temperature and process changes. However, the BiCMOS process used in constructing this circuit is expensive to implement, and resistors are also required in this implementation.
Consequently, a need arises for a brown-out detector circuit that is relatively insensitive to temperature variation over a wide temperature range, is economical to manufacture both in terms of process cost and in conservation of valuable integrated circuit area, and that demonstrates a predictable and benign response to process variations.